Many processes for fabricating semiconductors, such as etching and deposition, are plasma-enhanced or plasma-assisted; that is, they employ process reagents excited to a plasma state within a vacuum chamber. Generally, the plasma is excited by coupling radio frequency (RF) electrical power to the process gas mixture. The RF electrical field dissociates atoms in the process gas mixture to form the plasma.
One method of coupling RF power to the process gases is inductive coupling, in which an RF power supply is connected to an induction coil which is mounted either inside the chamber or just outside a portion of the chamber wall which is dielectric. In comparison with a capacitively coupled plasma source, an advantage of an inductively coupled plasma source is that it permits adjusting the IF power supplied to the plasma independently of the DC bias voltage on the semiconductor workpiece.
Induction coils commonly are shaped as a solenoid which either encircles the cylindrical side wall of the vacuum chamber, or else is mounted on the circular top wall of the chamber. Other conventional induction coils are shaped as a planar or semi-planar spiral mounted on the circular flat or dome-shaped top wall of the chamber. The solenoid and spiral coils share the disadvantage of producing an IF electromagnetic field which extends along the axis of the coil toward the semiconductor workpiece. A large RF field near the workpiece can be undesirable because it possibly can damage the semiconductor devices being fabricated on the workpiece.
U.S. Pat. No. 5,435,881 issued Jul. 25, 1995 to Ogle discloses an inductively coupled plasma source which minimizes the RF magnetic field near the semiconductor workpiece. It employs an array of induction coils distributed over the dielectric, circular top wall of a process chamber. The axis of each coil is perpendicular to the chamber top wall and to the semiconductor workpiece, and adjacent coils are connected out of phase so as to produce opposite polarity magnetic fields. This arrangement produces a "cusp" magnetic field pattern in the "near field" adjacent the top wall, which excites the process gases to a plasma state. However, in the "far field" near the workpiece, the opposite polarity magnetic fields cancel out so that the magnetic field strength near the workpiece is negligible, thereby minimizing any risk of damage to the semiconductor devices being fabricated.
One disadvantage of the Ogle design is that the RF magnetic field is non-uniform near the perimeter of the induction coil array. Specifically, the perimeter of Ogle's magnet array deviates from the central pattern of evenly spaced, alternating polarity, magnetic poles. Such spatial non-uniformity in the RF field can produce undesirable spatial non-uniformities in the plasma-enhanced semiconductor fabrication process.